1. Field of the Invention
This invention relates to integrated circuit fabrication and, more particularly, to a chemical vapor deposition system employing a thermocouple mounting system designed to inhibit contact between the thermocouple and a quartz liner within the chemical vapor deposition system.
2. Description of the Related Art
Chemical vapor deposition ("CVD") is a well-known process employed during the fabrication of an integrated circuit to deposit a thin film upon a substrate. A CVD process typically involves forming a non-volatile solid film (e.g., silicon dioxide, silicon nitride, polycrystalline silicon) on a substrate by reacting vapor phase chemicals that contain the required constituents. For example, silicon dioxide may be formed upon a semiconductor substrate by reacting silane and oxygen or by thermally decomposing tetraethylorthosilicate (TEOS). The thin film is formed by introducing the reactant gases into a reaction chamber and then decomposing the reactants and reacting them at a heated surface. Various inert carrier gases (e.g., H.sub.2, N.sub.2, Ar) may be used to carry the reactive gases into the chamber. The gaseous by-products of the reaction are desorbed and removed from the reaction chamber, along with the unconsumed reactant gases and the inert carrier gases.
The CVD process can take place in either pressurized or non-pressurized reaction chambers. Due to the stringent requirements of film uniformity, low-pressure chemical vapor deposition ("LPCVD") reactors have gained in popularity. LPCVD reactors generally operate in the pressure range of 0.1 to 10 torr and the temperature range of 500 to 600.degree. C. As such, the rate at which a solid film is formed at the surface of a semiconductor substrate is typically limited by the rate at which the reactant gases react rather than by the rate at which the reactant gases are supplied to the substrate by mass transport. By eliminating mass-transfer constraints on reactor design, the reactor may be optimized for high wafer capacity. In addition, low-pressure operation decreases gas-phase reactions, making LPCVD films less subject to particulate contamination.
Surface reaction rate is very sensitive to temperature, as shown by the following equation: EQU R=R.sub.0 e.sup.(-Ea/kT)
in which R is the rate of reaction, R.sub.0 is the frequency factor, E.sub.a is the activation energy in eV, k is Boltzmann's constant, and T is the temperature in Kelvin. As such, precise temperature control is essential in an LPCVD reactor. Typically, the temperature control system receives data from thermocouples and adjusts power to furnace heating elements to maintain the temperature at a predetermined set point. Modem systems are capable of controlling temperatures over the range of about 300-1200.degree. C. to an accuracy of about .+-.0.5.degree. C. over a length of up to 40 inches (the "flat zone").
Horizontal tube reactors are commonly used as LPCVD reactors because of their superior economy, throughput, uniformity, and ability to accommodate large-diameter (e.g., 150 mm) wafers. Horizontal reactors are, however, susceptible to particulate contamination of wafers placed in them. Wafers are aligned vertically and stacked in quartz racks or "boats" that support the wafers. A fused silica paddle supports the boats and is used to position the boats within the reactor. Considerable particle generation can occur when boat-laden paddles are dragged along the furnace tube during loading and unloading. The particles can land on the wafers and result in defects if particles become embedded in the growing film. The use of wheeled carriers can serve to somewhat reduce the generation of particles, but friction at the wheel bearings and movement of the wheels over the tube surface can still generate particles.
Greater reduction in the number of generated particles can be achieved by using suspended loading systems. In fully suspended loading systems, the boats and paddles are suspended at the end of a motor-driven rod and pushed into the furnace without touching the process tube walls. During processing the wafers remain suspended, and upon completion of processing the wafers are removed from the reactor, again without touching the walls of the tube. Soft-landing systems carry the boats into the process tube, lower the boats until the tube supports them, and then withdraw, leaving behind the boats and wafers. The paddles may remain within the tube or be withdrawn. Upon completion of processing, the boats and wafers are removed from the tube without touching the tube walls.
A recent innovation in furnace technology is the vertical furnace. In a vertical furnace, the wafers are also stacked side-by-side but are oriented horizontally rather than vertically (as in horizontal furnaces). The wafers are placed in boats or in perforated-quartz cages. The vertical orientation inhibits contact between the boats and the tube walls, and thus the formation of particles, without the use of suspended loading systems. Use of a vertical furnace may not completely eliminate particle formation, however. Vertical furnaces may include a quartz liner placed between the wall of the reactor and the quartz boat holding the wafers. The quartz liner may be used to confine the process gases in close proximity to the wafers during film formation. The thermocouple used to measure temperature within the furnace often includes an elongated housing placed between the wall of the furnace and the quartz liner with very close tolerance. If the thermocouple is misaligned, contact between the thermocouple and the quartz liner may cause formation of quartz particles that can contaminate the wafers. Further, such misalignment can result in incorrect temperature profiles because the temperature is being measured farther from the heating elements than called for by the furnace design criteria.
As an example, the Model Alpha 585S LPCVD reactor manufactured by Tokyo Electron Limited (Tokyo, Japan) includes a thermocouple housing inserted through an opening in the sidewall of the reactor and secured in place by an O-ring. FIG. 1 depicts a cross-sectional view of the reactor. Thermocouple 14 is inserted through manifold 16 and resides between sidewall 12 of reactor 10 and quartz liner 20. FIG. 2 is an enlarged view of the circled portion of FIG. 1. Thermocouple 14 is secured to manifold 16 by thermocouple mounting system 11 as follows: O-ring 22 is placed over the end of thermocouple 14 and secured in place with O-ring compression ring 24 and thermocouple mounting hub 26 to form a seal to preserve vacuum when reactor 10 is evacuated. Manifold 16 and thermocouple mounting hub 26 include threaded portions 18 and 28, respectively, that are complementarily threaded to form an engagement when thermocouple mounting hub 26 is screwed onto manifold 16. Clip ring 32 is then placed over notch 34 in the end of thermocouple 14, and thermocouple mounting bushing 36 is coupled to the mounting hub. Thermocouple mounting hub 26 and thermocouple mounting bushing 36 include threaded surfaces 32 and 40, respectively, that are complementarily threaded to form an engagement when thermocouple mounting bushing 36 is screwed onto thermocouple mounting hub 26.
As currently configured, thermocouple 14 is held in place essentially only by the engagement formed between manifold 16, O-ring 22, compression ring 24, and mounting hub 26. The design of thermocouple mounting bushing 36 allows only a weak, if any, engagement between the bushing and clip ring 32. Consequently, thermocouple 14 may wobble or move within manifold 16, and thermocouple 14 may shift as much as 2 or 3 inches at the end opposite manifold 16. As a result, contact between thermocouple 14 and quartz liner 20 may dislodge quartz particles from the boat and contaminate wafers contained within reactor 10.
In addition, when reactor 10 is used as part of a solvent-based TEOS system, the solvent may lubricate O-ring 22 and render thermocouple 14 mobile when the system is placed under vacuum. As such, the evacuation process may pull thermocouple 14 into quartz liner 20, cracking the quartz liner and necessitating replacement of one or both components. Further, if sufficient vacuum is created before O-ring 22 fails, the pressure differential between the exterior and the interior of reactor 10 may be sufficient to push thermocouple 14 completely into the reactor, thus voiding the vacuum and causing rupture or implosion of the reactor. Catastrophic failure of the reactor could result not only in economic losses due to loss of production capacity and the need to replace the equipment, but potentially in injury to workers near the reactor when the failure occurred.
It would therefore be desirable to develop a thermocouple mounting system that overcomes deficiencies of the current system. In particular, an improved thermocouple mounting system would include at least two points of contact between the thermocouple and the wall of the reactor. Such a configuration would inhibit wobbling or motion of the thermocouple within the reactor so that accurate temperature profiles are obtained and formation of quartz particles is inhibited. The improved mounting system would also be configured to maintain engagement between the thermocouple and the reactor in the event of O-ring failure.